With urea-formaldehyde (UF) resin as walls and capsaicin as core substances, microcapsules were prepared based on in situ polymerization process. The morphology and size distribution of the microcapsules were analyzed by Fourier transform infrared spectroscopy, laser particle size analyzer, and scanning electron microscopy. The microcapsulated capsaicin (MC) agents had a mean diameter of about 30-50 mu m. Moreover, the thermal properties of the MC agents were measured by differential scanning calorimetry and thermogravimetric analysis. It was demonstrated that the melting point and thermal stability of the MC agents were greatly improved compared with that of the uncovered capsaicin, which were Caused by the encapsulating crosslinked UF resin over the surface. The shell formation mechanism and the effects of the process conditions such as U/F ratio, shearing force, and acidification time on the particle size of the MC agents were discussed. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 116:2234-2241, 2010